1. Field of the Invention
The present invention relates to a tandem type brake power servo booster and a tandem type brake toggle joint therefor.
2. Description of the Prior Art
A valve body enclosing a valve mechanism is usually disposed in a center of a tandem type brake toggle joint. The tandem type brake toggle joint includes a front power piston and a rear power piston which are integrally connected to the valve body. As the valve body is located on the rear side of a center plate in the tandem type brake toggle joint, the rear power piston can be integrally attached to or formed with the valve body, but the front power piston, which is located on the front side of the center plate, must extend slidably through the center plate and be connected to the valve body. Therefore, a cylindrical member which slidably extends through the center plate is usually used to connect the front power piston to the valve body.
The cylindrical member is separate from the front power piston, and the front power piston has an inner periphery which is held between a shoulder and a nut on the cylindrical member. Thus, the front power piston and the cylindrical member are connected to each other. According to this arrangement, however, the front power piston is likely to rotate with the nut when the nut is tightened. A front diaphragm is disposed behind the front power piston, and its inner peripheral portion is interposed between the piston and the shoulder on the cylindrical member to define a seal between them. If the piston is rotated when the nut is tightened, the diaphragm is deformed or twisted, thus reducing its durability. Therefore, it has been necessary in the past to use a separate O-ring and to form an annular groove in the rear surface of the front power piston so that a bead of the diaphragm can be received in the annular groove.
One feature of the present invention is the provision of a tandem type brake toggle joint which includes means for preventing rotation of the front power piston and the cylindrical member so that the front diaphragm may define an effective sealing member.
Another feature of the present invention is the provision of a connecting structure which is located between a valve body and which encloses a valve mechanism for opening and closing a passage for a pressurized fluid in response to the axial movement of the input shaft. The connecting structure also encloses a hub which is disposed in front of the valve body. The connecting structure of the present invention receives a base end of a push rod.
In the prior art tandem type brake toggle joint, a valve body which encloses a valve mechanism and a hub in which the base end of a push rod is received are formed separately from each other. Subsequently, the valve body and the hub are integrally joined to each other. In a tandem type brake toggle joint, it is necessary to form or integrally join at least the valve body, a rear power piston and a front power piston. Moreover, it is necessary to employ a cylindrical member which extends slidably through a center plate and which divides a shell into a plurality of compartments. Various constructions have, therefore, been proposed to achieve these objects. Prior art devices which have a valve body and a hub that are separately formed from each other require a correspondingly large number of parts, and a correspondingly large number of proposals have been made for improving their construction. These constructions have a significant bearing on the ease of fabrication and the degree of freedom in design engineering.
A further feature of the present invention is that a valve body which encloses a valve mechanism for opening or closing a passage for a pressurized fluid in response to an axial movement of an input shaft, and a key member by which a valve plunger which forms a part of the valve mechanism is held in position in the valve body, are always kept apart from each other so that the valve body may be reduced in size and weight.
In a conventional tandem type brake toggle joint, the valve body which encloses the valve mechanism is formed separately from a hub in which the base end of a push rod is received. The hub has a base end which is fitted in the valve body, and the valve plunger is slidably fitted in the base end of the hub. A key member extends through the valve body and the hub and engages the valve plunger to hold it in position within the hub.
The valve body, which is formed from a synthetic resin, is likely to crack if its wall thickness is reduced in order to decrease the size and weight of a brake toggle joint. Detailed examinations have been made of the possible causes of cracking in the prior art tandem type brake toggle joint, and it has been found that the valve body cracks if a stepping force, which exceeds the level required to bring the brake toggle joint into a full load position, is applied to the valve plunger through the input shaft and transmitted to the valve body through the key member.
The last-mentioned aspect of the present invention is based on this discovery, and this aspect of the invention is characterized by the provision of the clearance between the key member and the valve body so that they may always be kept away from each other.